Heretofore, in order to control air-to-fuel ratio in internal combustion engines, various kinds of combustion apparatuses and the like, oxygen sensors for detecting oxygen concentration in exhaust gases have been used. As this kind of oxygen sensor, there has been known, for example, a sensor which measures oxygen concentration contained in a detection gas using a solid electrolyte which produces an electromotive force by a difference in oxygen concentration between a reference-gas side and a detection-gas side. The solid electrolyte is an oxygen-ion-conductive material, such as zirconia and the like. It is usually formed into the shape of a test tube or a cylinder, and a reference-electrode-side electrode and a detection-gas-side electrode are formed on its inner surface and outer surface, respectively.
On the surface of the detection-gas-side electrode, a porous protective layer of alumina is formed to protect the electrode. On a surface layer of the porous protective layer, particles of platinum catalyst for promoting the oxidation reaction of gas components are carried in order to detect air-to-fuel ratio with an excellent accuracy (refer to JP Patent Kokoku Publication No. 57-34900 (1982)).
That is, an oxygen-sensor element for controlling air-to-fuel ratio generally comprises a body of an oxygen-ion-conductive solid electrolyte and a pair of electrodes (a reference electrode and a measuring electrode) provided on its inner and outer surfaces, and is covered with a porous protective layer in order to protect the measuring electrode contacting exhaust gases from exhaust gases. In this kind of sensor element, however, excess air factor (.lambda.) shifts, that is, a so-called .lambda.-point shift is produced, due to unburnt components included within exhaust gases, and detection accuracy decreases. Various researches and proposals have therefore been done.
For example, an oxygen-sensor element carrying a noble-metal catalyst within a protective layer has been proposed (JP Patent Kokai Publication Nos. 53-50888 (1978), 50-14396 (1975) and 54-89696 (1979)).
Problems to be solved by the Invention
There has been a problem, however, that, when platinum is carried on the above-described protective layer (surface layer), gas components of a detection gas is excessively adsorbed on the platinum having high catalytic property, and the responsive property of the oxygen sensor decreases in some cases due to the adsorbed gases to produce a shift in the control of air-to-fuel ratio. In another example, there has been a problem that platinum particles penetrate into minute pores of the porous protective layer to produce clogging, and the responsive property of the oxygen sensor decreases.
Moreover, there has been a problem in durability. That is, carbon, CO and the like in a detection gas pass through the surface layer in some cases during the application. At this time, carbon is converted into graphite due to the action of the catalyst or the electrode to produce cracks in the porous protective layer, or to produce peeling in some cases. In the worst case, the protective layer peels off, and the electrode is sublimated. When the carried amount of the catalyst is increased, clogging is produced to cause deterioration in response. On the other hand, too small an amount of the catalyst causes dissipation of the catalyst to reduce its effect.
There has also been proposed an oxygen-sensor element which has two protective layers, and in which the catalyst is carried only in a layer disposed on the more outer side (JP Patent Kokai Publication Nos. 53-72686 (1978) and 55-13828 (1980)). In this kind of oxygen-sensor element, however, a catalyst-carrying layer itself easily peels off, and hence catalytic action can not be effectively utilized.
There has further been proposed an oxygen-sensor element comprising a material which occludes and releases oxygen as a protective layer (JP Patent Kokai Publication No. 62-245148 (1987)). However, this element also has the same problem as in the above-mentioned technique that the protective layer easily peels off, and deterioration in durability is also concerned about.